1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a high-breakdown-voltage power semiconductor device having a diode.
2. Description of the Background Art
In these years, inverters are used in those fields such as the field of industrial power units. For the inverter, usually a commercial power supply (AC power supply) is used. Thus, the inverter includes a converter unit first converting an AC voltage into a DC voltage (forward conversion), a smoothing circuit unit, and an inverter unit converting the DC voltage into an AC voltage (inverse conversion). As a main power device in the inverter unit, an insulated gate bipolar transistor (hereinafter referred to as “IGBT”) capable of performing a switching operation at a relatively high speed is chiefly applied.
In most cases, the load of the inverter is an electric induction machine (motor which is an inductive load). The inductive load is connected to a point of an intermediate potential between an upper arm element and a lower arm element, and electric current is caused to flow to the inductive load in both the positive and negative directions. Therefore, in order to direct the current flowing in the inductive load from the end to which the load is connected back to the power supply of a higher potential and to direct the current from the end to which the load is connected to the ground, a freewheel diode for circulating the current between the inductive load and a closed circuit of the arm elements is required.
In the inverter, usually the IGBT is operated as a switching device to repeat an OFF state and an ON state so as to control the power energy. Regarding the switching of the inverter circuit with an inductive load, the ON state is reached through a turn-on process while the OFF state is reached through a turn-off process. Here, the turn-on process refers to a transition of the IGBT from the OFF state to the ON state while the turn-off process refers to a transition of the IGBT from the ON state to the OFF state. While the IGBT is the ON state, current does not flow through the diode and the diode is the OFF state. In contrast, while the IGBT is the OFF state, current flows through the diode and the diode is the ON state.
In the following, a structure and an operation of a conventional diode will be described. In the conventional diode, an anode including a p-type diffusion region is formed in one main surface of an n-type low-concentration semiconductor substrate. In the other main surface of the semiconductor substrate, a cathode including an n+-type high-concentration region is formed. In order to ensure a breakdown voltage of the diode while a voltage is applied between the cathode and the anode, the diode including a guard ring (p-type region) is commonly and widely used. The guard ring is formed at a distance from the outer periphery of the anode (p-type diffusion region) so that the guard ring surrounds the anode, and accordingly an electric field in an anode's end portion located on the outer periphery side of the anode is reduced.
In the ON state where a high voltage is applied in the forward direction between the anode and the cathode, many carriers are accumulated in an n-type region (drift layer) of the semiconductor substrate. In contrast, in the OFF state where a high voltage is applied in the reverse direction between the anode and the cathode (at the time of reverse recovery), the carriers accumulated in the drift layer are discharged so that a reverse recovery current flows. At this time, a large current and a large voltage are applied to the diode, and accordingly heat is generated which is accompanied by large power consumption. This is one of the causes of hindrance to fast switching.
Documents disclosing a power semiconductor device having a diode are, for example, Japanese National Patent Publication No. 2011-514674, Japanese Patent Laying-Open No. 2000-114550, Japanese Patent Laying-Open No. 2003-101039, and Japanese Patent Laying-Open No. 07-221326.